Multiple Async Request Synchronization

Question

I'm developing a Silverlight app that makes multiple async requests to a number of web services. I want a modal "loading" dialog to stay active until all the requests have completed. I'm managing the situation by using a counter variable that gets incremented on each async request start event, and decrements on each async complete event (doesn't seem thread safe to me). When the counter is zero a property bound to the UI turns the dialog off. Is there a better/more general way of dealing with this problem than my counter solution?

Answer 1

Your counter solution is a valid one. Whatever you do, you will have to keep track of all your requests and understand when they arrive (when count hits zero).

You can do different things to clean up your code like put all of this implementation in some MultiAsyncWaiter class which returns an event when complete. But the fundamental implmentation will remain the same: keep track of them until they all return.

You are right about the thread unsafe-ness of the int. If you use interlocked operations (see comments) or lock on the variable, you can keep your implementation thread safe.

Why volatile keyword wont work: With multiple threads changing the variable, an interlocked operation is required for the decrement, which is technically a read + write operation. This is because another thread can change the value between the read and the write.